Machinery and electronic devices are becoming more and more sophisticated at an extremely rapid rate. An important element of this sophistication is the need to join metal to metal and metal to ceramic parts in more reliable and reproducible methods, often at high temperature (over 1000.degree. C.) and often without the use of flux or other chemical cleaning processes. A proliferation of techniques have been developed to meet these needs.
These include but are not limited to:
high temperature vacuum and inert gas furnace brazing PA0 electron beam brazing and welding PA0 laser beam brazing and welding PA0 spot welders and brazers PA0 ultrasonic bonders PA0 heli-arc machines PA0 acetylene torches PA0 mechanical bonding using electromagnetic forces PA0 soldering irons and guns PA0 plasmatrons or plasmajets PA0 ion beams PA0 require specialized, expensive equipment PA0 require highly trained personnel to operate, i.e. cannot be machine programmed PA0 do not give reproducible results PA0 require the use of chemical cleaning agents which can contaminate critical compounds and limit the lifetime of the device PA0 each new joint must be developed through the use of many samples which can be very expensive and time consuming PA0 lack of temperature control can cause unacceptable thermal stresses, unacceptable recrystallization in components, damages to neighboring components.
For the most part, these developments have been empirical. Many of them also have one or more of the following undesirable characteristics:
In any brazing or welding operation some provision must be made to accomplish the following processes:
(i) mechanically join or position the parts to be brazed or welded
(ii) choose an optimum braze material
(iii) properly position the right amount of braze and weld material or arrange to feed it at the proper rate during brazing or welding
(iv) establish a non-corrosive atmosphere
(v) clean, selectively if possible, the surfaces that are to be joined
(vi) if the parts are to be brazed, wet or coat the appropriate surfaces with a thin layer of material, usually the same as the braze material
(vii) heat the desired surfaces to the optimum temperature at the optimum rate.
(viii) ensure that the braze or weld material flows to the desired location
(xi) cool the joint at an optimum rate.
Ensuring that the steps outlined above are done in the proper sequence is extremely important in most cases. For instance, if the braze melts and flows before the surfaces are cleaned and wetted, it will ball up and often fall off of the metal parts or move to some undesirable position. There is little or no problem associated with accomplishing steps i and iv in proper sequence. However, steps v through viii are often all accomplished by applying heat to the joint. Ensuring that these steps are effected in the proper sequence becomes one of the prime requirements of the heating system. There is, however, some flexibility in the sequencing of these steps.
Besides ensuring the proper time sequencing of the steps noted above, the steps are strongly coupled so that any change in one step could strongly influence any of the other steps. For instance, heating a vacuum furnace rather than a hydrogen furnace could alter the decisions that must be made to properly accomplish steps ii, v, vi, vii, viii, and ix.
Until now, the physics and physical processes that determine the multiple interactions and proper sequencing of events during brazing or welding often have not been identified or even well understood. This is especially true in cases where an electrical discharge is used to supply the heat, because of the notorious "perversity" of arc discharges. Great strides, however, have been made in understanding and optimizing arc electrical discharge phenomena over the past 20 years. Many of these developments were accomplished in areas of technology not familiar to the average arc welding expert, e.g. space propulsion engines. Often, these developments were not reported in the open literature, hence this information was confined to a small group of plasma physicists. This can account for the almost universal use of "primitive" arc devices and technology in most present day heli-arc operations, as well as heli-arcing being more a "black art" rather than a well established technological process.
As stated above, rapid developments in other areas of technology as well can have profound implications towards the development of a scientific approach to welding and brazing. Specifically, the significant advances in our understanding of physi- and chemi-sorbed layers and how to control them have been made by scientists working in the space program. High speed, high capacity computers have made the solution of complicated heat-transfer problems routine. The development of solid state devices and associated electronics now permit better than 1% control of current, voltage or power in direct current power supplies.